Separation of CF4 and C2F6 from a perfluorocompound mixture

ABSTRACT

A process for separating at least one of CF 4  and C 2 F 6  from a gas. The process includes the steps of: 
     (a) contacting a gas mixture comprising (i) at least one of CF 4  and C 2 F 6 , (ii) at least one of NF 3 , CHF 3 , and N 2 , and (iii) SF 6  with a membrane at conditions effective to obtain a retentate stream rich in SF 6  and at least one of CF 4  and C 2 F 6 , and a permeate stream rich in at least one of NF 3 , CHF 3 , and N 2 ; and 
     (b) contacting the retentate stream with an adsorbent at conditions effective to adsorb SF 6  and produce a product stream rich in at least one of CF 4  and C 2 F 6 .

FIELD OF THE INVENTION

The present invention generally relates to a gas separation process. Theinvention particularly relates to a process for separating CF₄ and C₂F₆from a perfluorocompound gas mixture by a hybrid system involvingmembrane and adsorption separation techniques.

BACKGROUND OF THE INVENTION

Various fluorinated hydrocarbon gases including tetrafluoromethane (CF₄)and hexafluoroethane (C₂F₆) are used in the semiconductor industry toetch silica materials for use in integrated circuits. A major use ofC₂F₆, for example, is as a plasma etchant in semiconductor devicefabrication. Gases of high purity are critical for this application. Ithas been found that even small amounts of impurities in the etchant gascan increase the defect rate in the production of these integratedcircuits. Thus, there has been a continuous effort in the art to providea relatively simple and economical process for producing etchant gaseshaving minimal amounts of impurities.

One source of such etchant gases, of course, is the exhaust or vent gasfrom a semiconductor plasma etching process. The exhaust gas oftencontains unreacted CF₄ and/or C₂F₆, and other perfluorocompounds (PFCs)such as SF₆, NF₃, and CHF₃ as well as N₂. The exhaust gas is usuallyrecovered from the plasma etching process and concentrated from a fewparts per million to above 90% by volume in a PFC recovery stage. Thisconcentrated exhaust gas is sometimes referred to as a PFC mixture or aPFC soup. The PFC mixture normally contains about 90% by volume of CF₄and/or C₂F₆, and about 10% by volume of N₂, SF₆, NF₃, and CHF₃.

One way of purifying the PFC mixture to obtain substantially pure CF₄and/or C₂F₆ is by cryogenic distillation. However, there are somedrawbacks to such a process. Cryogenic distillation often requiresspecial equipment and has high utility costs. In addition, the PFCmixture is difficult to separate by cryogenic distillation due to thephysical properties of the gaseous components themselves; e.g., CF₄ andNF₃, and C₂F₆ and CHF₃ form an azeotropic mixture with each other.

It is also known in the art to use activated carbon or zeolites toremove chlorotrifluoromethane (CClF₃) and/or fluoroform (CHF₃) fromC₂F₆. See, e.g., U.S. Pat. No. 5,523,499. However, this adsorptionprocess is not said to be able to remove N₂, SF₆, and/or NF₃ from a gasmixture such as PFC soup.

Thus, it is an object of the present invention to address this need inthe art by providing a process that can separate CF₄ and/or C₂F₆ from aPFC mixture.

This and other objects of the invention will become apparent in light ofthe following specification, and the appended drawing and claims.

SUMMARY OF THE INVENTION

The present invention relates to a process for separating at least oneof CF₄ and C₂F₆ from a gas. The process comprises the steps of:

(a) contacting a gas mixture comprising (i) at least one of CF₄ andC₂F₆, (ii) at least one of NF₃, CHF₃, and N₂, and (iii) SF₆ with amembrane at conditions effective to obtain a retentate stream rich inSF₆ and at least one of CF₄ and C₂F₆, and a permeate stream rich in atleast one of NF₃, CHF₃, and N₂; and

(b) contacting the retentate stream with an adsorbent at conditionseffective to adsorb SF₆ and produce a product stream rich in at leastone of CF₄ and C₂F₆.

In a preferred embodiment, the present invention relates to a processfor separating both CF₄ and C₂F₆ from a gas. The process comprises thesteps of:

(a) contacting a gas mixture comprising CF₄, C₂F₆, NF₃, CHF₃, N₂, andSF₆ with a membrane at conditions effective to obtain a retentate streamrich in SF₆, CF₄, and C₂F₆, and a permeate stream rich in NF₃, CHF₃, andN₂; and

(b) contacting the retentate stream with an adsorbent at conditionseffective to adsorb SF₆ and produce a product stream rich in CF₄ andC₂F₆.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a process according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a process for separating at least one ofCF₄ and C₂F₆ from a gas mixture. The gas mixture preferably comprises(i) at least one of CF₄ and C₂F₆, (ii) at least one of NF₃, CHF₃, andN₂, and (iii) SF₆. More preferably, the gas mixture is an exhaust orvent gas from semiconductor fabrication process which comprises CF₄,C₂F₆, NF₃, CHF₃, N₂, and SF₆. The exhaust gas preferably has undergonean initial recovery or concentration step.

The gas mixture to be separated preferably comprises from about 10 toabout 95% by volume of at least one of CF₄ and C₂F₆. The balance of thegas mixture preferably comprises SF₆ and at least one of NF₃, CHF₃, andN₂.

The process according to the present invention comprises the steps ofcontacting the gas mixture with a membrane at conditions effective toobtain a retentate stream rich in SF₆ and at least one of CF₄ and C₂F₆,and a permeate stream rich in at least one of NF₃, CHF₃, and N₂; andcontacting the retentate stream with an adsorbent at conditionseffective to adsorb SF₆ and produce a product stream rich in at leastone of CF₄ and C₂F₆.

As used herein, the term “rich” means that the concentration of aparticular component in that stream is greater than the concentration ofthe same component in the feed stream to that process step. Likewise,the term “depleted” means that the concentration of a particularcomponent in that stream is less than the concentration of the samecomponent in the feed stream to that process step.

Any membrane can be used in the process of the present invention so longas the membrane can selectively retain SF₆ and at least one of CF₄ andC₂F₆ while passing the other components in the gas mixture through. Themembrane should also be substantially non-reactive with the gaseouscomponents to be separated.

Membranes suitable for use in the present invention include glassymembranes such as polymer membranes made preferably from polyimides;polyamides; polyamide-imides; polyesters polycarbonates; polysulfones;polyethersulfone; polyetherketone; alkyl substituted aromaticpolyesters; blends of polyethersulfone, aromatic polyimides, aromaticpolyamides, polyamides-imides, fluorinated aromatic polyimide,polyamide, and polyamide-imides; glassy polymeric membranes such asthose disclosed in U.S. Ser. No. 08/247,125 filed May 20, 1994, thecontent of which is hereby incorporated by reference; celluloseacetates; and blends thereof, copolymers thereof, substituted polymers(e.g. alkyl, aryl) thereof and the like.

Other membranes suitable for use in the present invention includeasymmetric membranes. Asymmetric membranes are prepared by theprecipitation of polymer solutions in solvent-miscible nonsolvents. Suchmembranes are typified by a dense separating layer supported on ananisotropic substrate of a graded porosity and are generally prepared inone step. Examples of such membranes and their methods of manufactureare disclosed in U.S. Pat. Nos. 4,113,628; 4,378,324; 4,460,526;4,474,662; 4,485,056; 4,512,893; 5,085,676; and 4,717,394; allincorporated herein by reference. The '394 and '676 patents disclosepreparation of asymmetric separation membranes from selected polyimides.Particularly preferred membranes are polyimide asymmetric gas separationmembranes as disclosed in the '676 patent.

Yet other membranes suitable for use in the present invention includecomposite gas separation membranes. These membranes typically have adense separating layer on a preformed microporous substrate. Theseparating layer and the substrate are usually different in composition.Composite gas separation membranes have evolved to a structure of anultrathin, dense separating layer supported on an anisotropic,microporous substrate. These composite membrane structures can beprepared by laminating a preformed ultrathin dense separating layer ontop of a preformed anisotropic support membrane. Examples of suchmembranes and their methods of manufacture are disclosed in U.S. Pat.Nos. 4,664,669; 4,689,267; 4,741,829; 2,947,687; 2,953,502; 3,616,607;4,714,481; 4,602,922; 2,970,106; 2,960,462; 4,713,292; 4,086,310;4,132,824; 4,192,824; 4,155,793; and 4,156,597; all incorporated hereinby reference.

Alternatively, composite gas separation membranes may be prepared bymultistep fabrication processes, wherein first an anisotropic, poroussubstrate is formed, followed by contacting the substrate with amembrane-forming solution. Examples of such methods are described inU.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994; all incorporatedherein by reference.

U.S. Pat. No. 4,756,932 describes how composite hollow-fiber membranesmay also be prepared by co-extrusion of multiple polymer solutionlayers, followed by precipitation in a solvent-miscible nonsolvent.

The membrane used in the present invention can be post-treated with, orcoated by, or co-extruded with, a fluorinated or perfluorinated polymerlayer in order to increase its ability to withstand harmful constituentsin the gas mixture from which PFCs are to be separate, at low levels ortemporary contact with such components.

The temperature of the gas mixture and/or the membrane during thecontacting step can vary from about −10° C. to about 100° C. Preferably,the temperature is between about 10° C. and 80° C. More preferably, thetemperature ranges from ambient, i.e., from about 20° C. to 25° C., toabout 60° C.

It is preferred, according to the present invention, to have a pressuredrop across the membrane of less than about 2,000 psig. More preferably,the pressure drop ranges from about 3 to about 200 psig. Even morepreferably, the pressure drop is about 20 to about 60 psig.

The requisite pressure drop across the membrane can be provided in oneof two ways. First, the feed gas stream can be compressed. Preferredcompressors are sealed and oil-free, such as the compressors sold underthe tradename POWEREX, available from the Powerex Harrison Company ofOhio. Second and more preferably, the pressure drop across the membranecan be established by lowering the pressure on the permeate side of themembrane. To create the lower pressure on the permeate side, a vacuumpump or any other suction device can be used.

The flowrate of the gas mixture across the membrane can vary from about0 to about 10⁵ Nm³/h per square meter of membrane available forseparation. Preferably, the flowrate is from about 10⁻⁴ to about 10Nm³/h-m². More preferably, the flowrate is from about 0.01 to about 0.5Nm³/h-m².

The membrane separation step preferably yields a retentate streamcomprising from about 60 to about 99% of at least one of CF₄ and C₂F₆,and from about 0.5 to about 4% of SF₆. The retentate stream may alsocontain trace amounts of NF₃ and CHF₃. This trace amount of impuritiesmay be removed in a subsequent adsorption unit. The membrane separationstep also preferably produces a permeate stream comprising from about 10to about 60% by volume of at least one of NF₃, CHF₃, and N₂.

The adsorption step in the process of the present invention can becarried out by either pressure swing adsorption (PSA) or thermal swingadsorption (TSA). Both adsorption techniques are well known in the art.This step can be carried out with the adsorbent in a packed bed, movingbed, or fluidized bed.

The adsorption step may be conducted at a pressure ranging from 50 to1.5 bar, and preferably from 20 to 3 bar. From an economic standpoint,the adsorption pressure is mostly dictated by the membrane retentatestream pressure. The temperature for carrying out this step can varyfrom 30° to 100° C. The flowrate per unit adsorbent (i.e., spacevelocity) can vary from 20 min⁻¹ to 0.1 min⁻¹, and preferably from 10min⁻¹ to 1 min⁻¹.

Any adsorbent can be used in the process according to the presentinvention so long as the adsorbent can selectively adsorb SF₆ from a gasstream comprising CF₄ and C₂F₆. Suitable adsorbents include zeolites,activated carbons, carbon molecular sieves, and polymeric adsorbentresins.

Preferably, the zeolite used in the present invention has a silica toalumina molar ratio of 1:1 to 100:1, and more preferably, of 1:1 to50:1. Even more preferably, the zeolite is an X-type zeolite. Prior touse, the zeolite should be ion-exchanged with Ca, Na, Li, Li/Zn, Be, Mg,or Fe. Exemplary ion-exchanged, X-type zeolites include NaX zeolite, CaXzeolite, and LiX zeolite.

Various commercially available activated carbons may be used in thepresent invention including BPL, F-300, F-400, and PCB from Calgon, BACfrom Union Carbide, and RB2 from Norit. PCB activated carbon sold byCalgon is preferred.

Similarly, various commercially available polymeric adsorbent resins mayused in the present invention. An exemplary polymeric adsorbent resinthat is suitable for use in the present invention is DOWREX, which issold by Dow Chemical Company.

The amount of adsorbent used, of course, varies depending on the amountof impurities to be separated and the desired purity of the product gas.Such a determination is within the scope of one skilled in the art.

Following use, the adsorbent is usually regenerated by desorption of theadsorbed impurities such as SF₆. Various methods are known in the artfor desorbing impurities adsorbed onto the adsorbent. Generally,desorption can be effected by changing any thermodynamic variable whichis effective in removing the adsorbed components. For example,desorption may be carried out using a thermal swing cycle, a pressureswing cycle, or a vacuum cycle; all of which are known in the art.Alternatively, the adsorbed components may be removed by using astripping gas or liquid. The stripping material may be one of theprocess feed materials or another material such as N₂, He, Ar, or steam.

The conditions for carrying out the regeneration step and the amounts ofstripping material, if employed, can be readily determined by one ofordinary skill in the art.

The recovery of CF₄ and/or C₂F₆ from the gas mixture can be furtherincreased either by increasing the number of separation stages or byincorporating one or more feedback (recycle) loops. Such modificationsare within the scope of the present invention.

Furthermore, the process according to the present invention can beemployed in combination with a cryogenic distillation column to producehigh purity CF₄ and/or C₂F₆. It can also be installed on-site of asemiconductor fabrication facility with a typical PFC recovery unit.This would reduce the shipping volume for off-site purification.

FIG. 1 is a flow diagram of a preferred process of the presentinvention. A feed gas stream 1 comprising CF₄, C₂F₆, NF₃, CHF₃, N₂, andSF₆ is introduced into a membrane separation unit 10. The feed gasstream 1 is contacted with a membrane at conditions effective to obtaina retentate stream 2 rich in SF₆, CF₄, and C₂F₆, and a permeate stream 3rich in NF₃, CHF₃, and N₂. The retentate stream 2 is then passed to anadsorption unit 20 in which SF₆ and any remaining NF₃ and CHF₃ areadsorbed. The adsorption unit 20 produces a product stream 4 rich in CF₄and C₂F₆.

While the invention has been described with preferred embodiments, it isto be understood that variations and modifications may be resorted to aswill be apparent to those skilled in the art. Such variations andmodifications are to be considered within the purview and the scope ofthe claims appended hereto.

What is claimed is:
 1. A process for separating at least one of CF₄ andC₂F₆ from a gas, said process comprising the steps of: (a) contacting agas mixture comprising (i) at least one of CF₄ and C₂F₆, (ii) at leastone of NF₃, CHF₃, and N₂, and (iii) SF₆ with a membrane at conditionseffective to obtain a retentate stream rich in SF₆ and at least one ofCF₄ and C₂F₆, and a permeate stream rich in at least one of NF₃, CHF₃,and N₂; and (b) contacting said retentate stream with an adsorbent atconditions effective to adsorb SF₆ and produce a product stream rich inat least one of CF₄ and C₂F₆.
 2. The process according to claim 1,wherein said gas mixture comprises both CF₄ and C₂F₆, and said retentatestream and said product stream are rich in both CF₄ and C₂F₆.
 3. Theprocess according to claim 1, wherein said gas mixture comprises NF₃,CHF₃, and N₂, and said permeate stream is rich in NF₃, CHF₃, and N₂. 4.The process according to claim 1, wherein said conditions in step (a)comprise a temperature between about 10 and about 80° C., a pressuredrop between about 3 and about 200 psig, and a flowrate rate betweenabout 10⁻⁴ and about 10 Nm³/h-m².
 5. The process according to claim 1,wherein said membrane is selected from the group consisting ofpolyimides, polyamides, polyamide-imides, polyesters, polycarbonates,polysulfones, polyethersulfone, polyetherketone, alkyl substitutedaromatic polyesters, and blends of polyethersulfone, aromaticpolyimides, aromatic polyamides, polyamides-imides, fluorinated aromaticpolyimide, polyamide, and polyamide-imides.
 6. The process according toclaim 1, wherein said adsorbent is a zeolite, activated carbon, orpolymeric adsorbent resin.
 7. The process according to claim 1, whereinthe temperature of the gas mixture during step (a) is from about 20° C.to 25° C.
 8. A process for separating CF₄ and C₂F₆ from a gas, saidprocess comprising the steps of: (a) contacting a gas mixture comprisingCF₄, C₂F₆, NF₃, CHF₃, N₂, and SF₆ with a membrane at conditionseffective to obtain a retentate stream rich in SF₆, CF₄, and C₂F₆, and apermeate stream rich in NF₃, CHF₃, and N₂; and (b) contacting saidretentate stream with an adsorbent at conditions effective to adsorb SF₆and produce a product stream rich in CF₄ and C₂F₆.
 9. The processaccording to claim 8, wherein said conditions of step (a) comprise atemperature between about 10 and about 80° C., a pressure drop betweenabout 3 and about 200 psig, and a flowrate rate between about 10⁻⁴ andabout 10 Nm³/h-m².
 10. The process according to claim 8, wherein saidmembrane is selected from the group consisting of polyimides,polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones,polyethersulfone, polyetherketone, alkyl substituted aromaticpolyesters, and blends of polyethersulfone, aromatic polyimides,aromatic polyamides, polyamides-imides, fluorinated aromatic polyimide,polyamide, and polyamide-imides.
 11. The process according to claim 8,wherein said adsorbent is a zeolite, activated carbon, or polymericadsorbent resin.
 12. The process according to claim 8, wherein thetemperature of the gas mixture during step (a) is from about 20° C. to25° C.